Striker Benteke, who has scored 21 goals since joining the club in 2016, has agreed a one-year extension to remain at Selhurst Park until the summer of 2021.
Defender Tomkins, who is currently sidelined with a groin injury, is now committed until the end of the 2021-22 campaign.
Chairman Steve Parish told the club's official website: "I'm pleased that agreements have been reached with these two important senior players.
"Both James and Christian bring vast Premier League experience, commitment and professionalism to the squad.
"These new deals show that we recognise their levels of performance in recent seasons, that we believe in both players, and that they are hugely motivated by the direction the club is heading."
Former Aston Villa and Liverpool forward Benteke has made 98 appearances for the Eagles.
He has netted just once this term after an injury-hit 2018-19 season.
The Belgium international said: "I am very happy to have reached an agreement to extend my contract with Palace for another year.
"The team have started the season really well and there is a lot to be positive about. I am more committed than ever to repay the faith that the chairman, manager and our amazing fans have shown in me."
Tomkins has played 87 times for Palace since his move from West Ham three years ago.
He has been out of action since April but is now nearing a return for Roy Hodgson's side, who are sixth in the Premier League after a fine start to the season.
Tomkins said: "I'm now in my fourth season and I've really enjoyed my time at Palace.
"I'm excited about the future here and didn't hesitate to commit myself to the club for an additional two years. It's been great to be part of this team and to have the support of the chairman, manager and staff."